This invention relates generally to helically wound multi-layer coils for use in compact fluid heaters, particularly of the type disclosed in U.S. Pat. No 3,282,257, specification and claims of which are incorporated herein by reference thereto. Also incorporated by reference herein is U.S. Pat. No. 2,771,934 pertaining to coil winding techniques.
Generally speaking, coils of the type disclosed herein are utilized in high efficiency compact fluid heaters wherein coils are arranged in layers intermediate a combustion system and exhaust stack. The structure of this type of unit utilizes the combustion system to generate high temperature products of combustion from burning fuels flowing axially through the centers of stacked coils and then radially through successive layers of each coil from the internal diameter through an external diameter of each coil in a given stack. In this manner, heat is extracted from the burner flue gases by convective heat transfer to each turn and layer of the coil bank as the flow of combustion products progresses from the internal diameter through the external diameter, whereupon it is exhausted to the heater stack or exhaust means.
Presently used coils are wound on a mandril as shown on U.S. Pat. No. 2,771,734 (reference FIG. 1) from continuous lengths of metal tubing appropriately chosen for the operating temperatures and pressures involved. Thus, as wound, the initial end of a single length of coil is embedded in the first layer of a given coil as shown in U.S. Pat. No. 3,282,357 (reference FIG. 3). Coils of this type work properly. However, a difficulty arises in that internal connections to the coil must be made with the connection located in the path of combustion products. This type of connection extends into the combustion products flow of the associated burner utilized in the heater (reference FIG. 1 of U.S. Pat. No. 3,282,357).
With this structure, replacing a single coil of a manifolded bank becomes difficult and expensive, since it is necessary to enter the boiler from one end and physically detach the internal connection. Subsequently, it is necessary to re-enter the boiler to re-connect an internal end on a replace coil. Associated difficulties arise in that the coil positioning must be precise in order to match the manifold opening in place.
The invention disclosed herein overcomes the above mentioned difficulties through the use of a bifilar design incorporating two lengths of tubing per coil. Individual lengths of suitable tubing are simultaneously wound into rows and layers as in the above described standard coil. As wound, however, the initial ends of each tubing length, embedded in the coil first layer are joined so as to be fluid communicating. Subsequent layers then precede with simultaneous winding of the remaining lengths of the two individual tubing lengths until a final or outer layer of the coil is formed, having the terminal ends of the initial two tubing lengths embedded in the outer layer.
With the type of construction disclosed herein, fluid flowing into one of the outer layer tubing ends traverses the first tubing length now wound into rows and layers of the coil, passing through the joined initial ends of both first and second tubing lengths, returning to the outer layer through the second tubing length now wound into rows and layers of the coil, exiting in the terminal end of the second tubing now embedded in the outer layer. This coil construction overcomes the above mentioned replacement difficulties. Also outer layer coil termination provides a convenient location for manifolding fluid flow into and out of the banks of bifilar wound coils since the manifold devices can be located entirely at the heater surface.
It is, therefore, an object of the invention to provide a multi-layer multi-turn coil for a compact fluid heater wherein fluid flowing into and out of the coils can be terminated at the outer coil surface.
It is an additional object of the invention to provide a coil structure for a compact fluid heater wherein the inner coil tubing terminations do not project into the combustion gas flow.
It is yet an additional object of the invention to provide a coil structure for a compact fluid heater wherein coils can be serviced without severing termination in the combustion gas zones.
It is a further object of the invention to provide a coil for a compact fluid heater wherein fluid flowing in portions of the coil tubing are in both counterflow and parallel flow relationship with regard to heat exchange from products of combustion.